1. Field of the Invention
This invention relates to an aircraft wing assembly including a leading edge high lift device and in particular to such assemblies incorporating means for reducing the chordwise extent of the turbulent boundary layer over at least one of the upper and lower wing surfaces. The invention also extends to aircraft including such wing assemblies, to high lift devices for use in such assemblies, and to methods of reducing the drag associated with a wing having a leading edge high lift device.
In this Specification, the terms forward, rearward, upper, lower, chordwise, spanwise, etc., refer to the orientation of an aircraft when in straight and level flight.
2. Discussion of the Prior Art
Great effort and expense has been expended by aircraft designers and aerodynamicists to reduce as much as possible the drag of the aircraft and particularly the drag experienced by the flow of air over the surface or skin of the aircraft. Typical measures have included making the surface very smooth providing the surface with a special texture or microscopic structure to enhance the characteristics of flow over the surface. Another technique is to apply a method of boundary layer control referred to as forced hybrid laminar flow. In this process, a laminar boundary layer is maintained over the wetted surface for as long as possible so as to delay the onset of a turbulent boundary layer. It is not normally feasible to prevent entirely the transmission from a laminar to a turbulent boundary layer, although by delaying its onset the boundary layer can be kept in the laminar regime for longer, thereby leading to a reduction in drag. The process is known as forced hybrid laminar flow as the principle aim is to delay the onset of turbulence rather than to maintain a completely laminar flow over the wetted surface (although we do not intend to exclude this possibility).
In order to achieve this form of boundary layer control, the boundary layer is caused to adhere to the surface by providing a negative pressure over the surface by means of a multiplicity of perforations--of the order of microns in diameter--in the surface. This reduces the growth rate of the boundary layer and thereby delays the onset of the laminar/turbulent transition.
One of the main problems encountered in the design of forced hybrid laminar flow aircraft wings is the need to provide voluminous ducting in the fixed leading edge of the wing to provide the suction required to draw air through the perforations in the wing surface. The ducting area required encroaches on the space normally occupied by the forward spar of the wing. Shifting the forward spar rearwardly to accommodate such ducting has tremendous implications in terms of the increased weight of the wing. Furthermore, for a wing with a leading edge slat, the area conventionally occupied by the extension/retraction and guide mechanisms for the slats would now be occupied by the voluminous ducting. Accordingly proposals to date for a leading edge slat design for a wing with forced hybrid flow over its upper surface have reverted to the Kruger flap which involves providing a hinge at the lower forward extremity of the fixed leading edge or "D nose" of the main wing and making a portion of the underside of the "D nose" hingeable downwards and forwards. However Kruger flaps are not as aerodynamically efficient as slats.
A need therefore exists for a wing assembly which incorporates a high lift device and a provision for forced hybrid laminar flow, but which does not compromise the structural design of the main wing portion and which does not have the weight penalties referred to above.